The present invention relates to a display device using a light emitting element of a self light emission type, such as an EL (electroluminescence) element that emits light by application of a voltage.
An EL element is a light emitting element having a great potential because light emission is produced at a relatively low voltage and manufacturing thereof is easy. An element life, which has been a problem in a practical use of the EL element, now reaches a practical level. Accordingly, the EL element is beginning to be adopted for an in-car audio system and a mobile telephone.
FIG. 6 shows a structure of a conventional EL element. As shown in FIG. 6, in the EL element, a transparent electrode 31 made of indium tin oxide (ITO) or the like, a hole transporting layer 32, a light emitting layer 33, and an electrode 34 made of aluminum or the like are laminated in succession on a substrate 1 made of glass or the like. Here, an EL element portion 11 is composed of the transparent electrode 31, the hole transporting layer 32, the light emitting layer 33, and the electrode 34.
According to the EL element, the transparent electrode 31 is used as an anode and the electrode 34 is used as a cathode. Holes and electrons which are injected from the anode and the cathode, respectively, are recombined in the light emitting layer 33, thereby emitting EL light 211. The EL light 211 transmits through the substrate 1 and is emitted externally of the EL element.
Other than the structure of the organic EL element shown in FIG. 6, there are various structures, for example, (1) a structure having an anode, a light emitting layer, and a cathode, (2) a structure having the anode, a hole transporting layer, the light emitting layer, an electron transporting layer, and the cathode, (3) a structure having the anode, the light emitting layer, the electron transporting layer, and the cathode, and (4) a structure having the anode, a hole injection layer, the hole transporting layer, the light emitting layer, and the cathode.
Also, FIG. 7 shows a conventional structure of another EL element. As shown in FIG. 7, in the EL element, a transparent electrode 31 made of ITO or the like, a hole transporting layer 32, a light emitting layer 33, and an electrode 35 made of aluminum or the like are laminated on a substrate 1. Here, an EL element portion 12 is composed of the transparent electrode 31, the hole transporting layer 32, the light emitting layer 33, and the electrode 35. According to the EL element, the transparent electrode 31 is used as the anode and the electrode 35 is used as the cathode. Holes and electrons which are injected from the anode and the cathode, respectively, are recombined in the light emitting layer 33, thereby emitting EL light 213. At this time, when a film thickness of the electrode 35 is sufficiently small (for example, 20 nm or less), EL light 212 transmits through the electrode 35 and is emitted externally of the EL element.
Also, FIG. 8 shows another structural example of a conventional EL element. In the EL element, a transparent electrode 31 made of ITO or the like, a hole transporting layer 32, a light emitting layer 33, an electron transporting layer 36, and an electrode 37 made of ITO or the like are laminated on a substrate 1. Here, an EL element portion 13 is composed of the transparent electrode 31, the hole transporting layer 32, the light emitting layer 33, an electron transporting layer 36, and the electrode 37. The EL element having such a structure has been proposed by J. Kido et al., Yamagata University. Here, the transparent electrode 31 is used as the anode and the electrode 37 is used as the cathode. Holes and electrons which are injected from the anode and the cathode, respectively, are recombined in the light emitting layer 33, thereby emitting EL light 214 and EL light 215 as shown in FIG. 8. The EL light 215 transmits through the substrate 1 and is emitted externally of the EL element. The EL light 214 transmits through the electrode 37 and is emitted externally of the EL element.
The EL element as shown in FIG. 7 or 8 is transparent in a state in which no voltage is applied thereto, and emits EL light when the voltage is applied thereto. A display device in which a light emission pattern is emerged on such a transparent background is called a see-through display device, and display devices for automobile use and the like have been proposed.
FIG. 4 shows an example of a double-sided display device in which the EL element shown in FIG. 6 is applied to a display device of a folding mobile telephone having a rear display device. The double-sided display device has a structure in which two EL elements, each of which is obtained by forming the EL element portion 2 on the substrate 1 and has a sealing structure 3 that shuts out air are laminated such that the surface of one sealing structure 3 is bonded to the surface of the other sealing structure 3. Each of the EL elements emits the EL light 211 by application of a voltage. Such a structure has a thickness corresponding to two EL elements. Therefore, there is a limitation with respect to a display device for mobile telephone use, for which a reduction in thickness is required.
FIG. 5 shows another example in which the EL element shown in FIG. 7 is applied to a double-sided display device of the folding mobile telephone having the rear display device. The double-sided display device is obtained by forming the EL element portion 12 on the substrate 1 and has a sealing structure 3 that shuts out air. When a program for a driver circuit is switched over, display contents resulting from the EL light 212 and the EL light 213 can be visually identified in both surfaces. However, there is the following problem with respect to the double-sided display device having such a structure. The problem will be described with reference to FIG. 11. An EL cell 100 shown in FIG. 11 has an EL element structure in which light is emitted from both surfaces as shown in FIG. 7 or 8. EL light 121 and EL light 123 are emitted from a light emitting portion 101 by application of a voltage. An observer 200 visually identifies a display content resulting from the EL light 121. In addition, external light 130 and external light 132 transmit through the EL cell 100, thereby respectively becoming transmission light 131 and transmission light 133. Further, a part of the external light 130 is reflected on a substrate surface of the EL cell 100 and in an inner portion thereof at several %, thereby becoming reflection light 134. Therefore, the observer 200 visually identifies the transmission light 133 and the reflection light 134 together with the EL light 121, so that there is a problem in that a contrast is reduced. In addition, the EL light 123 is emitted to an opposite side to the observer 200. Accordingly, for example, when the EL cell 100 is applied to the folding mobile telephone, there is a fear that a display content resulting from the EL light 123 is caught by others.
As described above, when the display device which can be observed from both surfaces is composed of the self light emitting display element, there is the problem in that the contrast is reduced.